I think Warmaster in 28mm is a great idea and something I've given thought to but have yet to execute. It's one of about 5 GW games I will deign to play and I think it would work great in 28mm. Cut out some 100mm x 50mm movement trays and put however many guys / tray and you're set. Depending on how big your armies are you could do 4 or 5/ tray or up to 8-10, giving units of up 24 25mm figures or 30 20mm figures, or between 6 and 12 cav. Convert inches to cm if your table size allows, but probably not necessary.