Screen printing on what? Cloth, paper? Or something else?
If you're creating spot colour separations from Photoshop and it's a smallish (within A4) document at 100%, then maybe doing a multi-channel document at 600-1200ppi would be best particularly if there's type or clean sharp-edged graphic shapes involved.
Output from Photoshop CS1 or higher through the Print Preview dialogue and keep a close eye on the output options screen frequency, angles, registration marks, emulsion orientation etc.
Depending on the surface and its characteristics, you may be able to get up a clean dot up to 65 lpi or possibly even higher.