For 2.5 dial setting, use upper scale (marked 0,5,10,15,20,25) and divide the reading by 10.
For 250 dial setting, use upper scale and multiply the reading by 10.
The idea is that the adjustment factor makes the full-scale (needle all the way over) reading the same value as the dial setting.
As for a digital meter. Presumably you've built your layout etc. and have been running trains fine without one so obviously it's not a must-have.
In fact, if you just want to mess around with one, I will send you a new Harbor Freight digital meter (the kind you cab get Free with coupon). Battery included! I'll even test it first and the postage is on me! I have a bunch of them.
I don't see your email in your profile so email me a shipping address to this temporary email address which I'll disable after I hear from you one way or the other:
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