suggestions for router lead

I am trying to design a small CNC router based around a Dremel or
similar. I have decided upon a fixed bridge design with Y axis on
the bed and X,Z on the bridge. I thought that this would give me a
nice stiff machine and maximise the force available from the Y
stepper.

Anyway, I'm looking into leadscrews. I am considering acme screws
with Delrin nuts but am having a hard time deciding on the lead I
should go for. Obviously more lead gives greater speed but
resolution suffers.

I want a reasonable swift machine that will do some basic pcb
milling, ply part cutting and mould making. The design is based on
an A4 paper sized bed and is designed to be sturdy as I intend to use
quite a lot of 1/2" ali in the construction. Steppers a 200 step
bipolars.

Cheers,

Graham Stabler
Nottingham, England.