Try holding the cast bullet in the micrometer and slowly rotating it. Do this several times and note the highest reading you get - it's not perfect, but you don't have to be within 1/10000" in these things. For the 303 Brit in a Lee Enfield, it's rare that a bullet smaller than 0.314" in diameter works best, so I'd aim at that. The Lee 90385, 90361 and 90371 all work well, and typically drop at 0.313" or 0.314".
Get to know these guys: http://www.ranchdogoutdoors.com/
They sell a bullet for the 32-20 that makes a good plinker and drops at 0.313" or more depending on your alloy
They also stock a Lee 0.313" lube-sizer - no need to order a custom one.
I recommend that mold, or any of the three Lee molds and a 0.314" sizer.
Lyman, RCBS and others make much higher quality molds, but they are priced accordingly. For someone just starting, those prices might be just high enough to turn you away from casting, much less experimenting. Nevertheless, the Lee molds will provide thousands of bullets before they crap out, certainly enough service for you to decide if you'd like to get more serious about casting.
I tried your measuring while spinning the slug trick and I get 0.313 (0.312 - 0.3135). I got 0.314 once and wasn't able to get it again so I figure it was just the way I was holding the slug.
So I guess I should go for the Lee .313 sizer eh?




















































