I just tried the scenario myself - it is quite possible to complete on auto-fireman, but gets so much easier with manual firing (if you know how to work it effectively). I found myself activating the water tower with 20 gallons left and the injectors on, so it was a very close thing.
The key thing to remember is that water consumption depends mostly on the Steam Generation Rate. It's tempting to strictly minimise this number in the F5 display, but you must balance that with the need to actually reach Evercreech. So you want the most miles per hour for the least pounds of steam generated (with apologies to Riddles). When going uphill, this is a very hard optimisation problem.
With manual firing, the first things you will want to do are to turn off the blower, open the dampers, and stop shovelling coal in. More coal on the fire means more steam generated, which is what you don't want too much of. With auto-fireman, you don't get these options.
The next step is to release the brakes and get going. About 55% regulator and full forward reverser will do for this, and don't forget to move the brake lever to Running as soon as the brakes are completely off (21.0") - the Release position consumes steam that you will need for climbing. (Why 55% regulator? Because Kuju screwed up and made the Black Five into a Compound, and nobody's fixed it since. At 50% regulator or less, the steam consumption and generation are much higher than above there, because many Compounds - including the LMS ones - use the first regulator position for simple working.)
As soon as you are properly moving, dial back to 45% reverser (if you're looking at the F5 display) or 35% (if you're looking at the F4 console), then advance to 100% regulator. This reduces steam generation and consumption, and uses your steam more efficiently. The line goes uphill from here quite steeply, so you will need to stay in these settings for quite some time. If you drop below 15mph, advance the reverser some more to keep moving.
Don't worry if the boiler pressure or your speed drops a bit. You want to stop the safety valves from lifting, because that is (hot) water spraying straight out into the air. The above reverser setting should get you up the worst of the hill, by taking advantage of some slacks in the gradient on the way.
If you're manual firing, you will need to turn the blower back on and close the firebox doors before entering tunnels, of which there are several on this route. You may also need to add coal at a slow rate (when not in tunnel) to retain enough steam to complete the climb. Keep the water up to about 50% using one injector only (the auto-fireman kicks in at 70% and refills to 90%!), which you might not need to do until you reach the summit.
When you pass Chilcompton, it's a good idea to reduce the reverser a bit, say to 40%/30%. There's still some hill left, but with auto-firing you will be *really* low on water by now, and you need to save every bit you've got. A little further on you can reduce to 25% or even 20% and still comfortably make the summit.
Just before you reach the summit proper, shut the regulator completely. You won't need *any* more steam until you reach Evercreech. With that in mind, shut the dampers to cool down the fire (you can do this in auto-fireman mode too, hold down Shift-M for a few seconds) and generally turn off everything you can. You should try to turn off everything you can think of, and just let your speed build up on the 1:50 down.
But don't forget the twin tunnels and their 30mph speed restriction. You'll need to brake for that, and use the blower for safety. Release the brake again once clear.
As long as you don't overdo the braking at the foot of the last hill, you can now coast all the way into Evercreech. The job is not yet over, though - you still have to save your engine, and some dolt put the water crane on the wrong side of the line.
(Real water cranes can reach locos standing on both lines if they are placed between them like that. Boo Kuju.)
It is at approximately this point that the ADHD auto-fireman with OCD might decide to turn on the injectors and completely drain the tank. So the last part might have to be don very quickly and accurately. With manual firing you can just eave the injectors off.
The crossover is on a hill which will tend to pull you away from the station. Anticipate this by selecting reverse before you reach it, and use counter-steam braking as a recognised shunting technique.
Use 100% reverser (each way) and small regulator positions for this relatively delicate manoeuvre. Counter-steam braking is also very useful for stopping sufficiently precisely so that your tender is under the crane. (Railworks doesn't care which part of the tender the crane points at. Real water cranes have some flexibility but not *that* much.) Use the normal brakes to remain stopped during the operation.
From this point on, you can be as extravagant as you like with the coal and water. Ironically, the rest of the journey is on almost level track...
