New MDF Door Linings / frames in old openings, how to get perfect linings fitted for new doors

My 76mm and 102mm Hinge jigs for sale at https://www.robc.co.uk​ My Amazon store for all kinds of goodies https://www.amazon.co.uk/shop/robincl... I like using MDF door linings and have been doing so for quite a while, especially as standard door linings no longer seem to fit!! I am experimenting with an MDF panel to represent a door and the gaps it will need for fitting to keep the lining perfectly true and straight while I fix them back to the existing openings With all clients and customers looking for nice factory finished doors now this enables you to be able to just chop in the hinges and hang the door with only the height needing adjustment if the floors are out like we have here