Following on from a post in the project announcement forum i thought it best to post here.
Took a gamble on a 2mm slot cutter for t-molding as all the posts i read were saying that you needed 1/16" (1.5875mm) cutter, seems to work perfectly in 18mm mdf and holds it very tight, guess its because its quite fibrous, might be a different story in plywood.
https://www.trenddirectuk.com/trend-c143bx1-4tc-2mm-slotting-cuttergot mine for less than £20 and with all trend tools its very sharp and will last longer then the chinese cheesemetal ones you see on ebay.
just used a scrap piece of MDF and eyeballed it centre for a test
The t-molding i got claimed to be 19mm but its more like 19.4mm using a Calliper, shouldn't really tell once its installed in a vinyl wrapped bartop