Hi Lee,
I did my recent roof tiles with Sheet Styrene
https://armorama.kitmaker.net/forums/58286&page=1Take a complete 0.25 mm (that's 0.010 '' i think) Evergreen sheet.
Fix it with tape on a flat surface (best is only a little bigger than the sheet ), a rectangular tile or something.
I you don't want to mark the ind. slates width with a pen, attach a metallic ruler along the sheet with a piece of tape so that it is fixed.
Now take a steel machinist square to make your first series of lines with an X-Acto, slow, with a new blade one pass is enough, it should not go through. The machinist square can slide along your tile and you are using the ruler for the measures, no need to mark the sheet.
For doing the other side, i leave the sheet fixed, mark my slates height or wide, and with the machinist square, do the same as above.
Before detaching the tape, pass with a sanding paper, to eliminate the elevations from the scribbing.
Take it away from the tile, prime it and that's it.
I have found out that there is no need to give it a first layer of colour, because after braking apart the individual tiles , the border's are still white so you need to fix them later.
Another tip : Do not glue the slates with normal plastic solvent, 0,25 mm is so thin, the bonding procedure wil be seen through the slate. Normal white wood glue is enough.
I know that this was some awfull english, so , if it was unclear , please ask back, and i also can make some pics tonight, because i am actually doing another roof myself.
By the way , my projects hotel , had some 4000 individual slates.
Cheers to Australia
Claude