Thanks for that. Had a look at your plaster tool method and noted that the tool is cast from the rough ground mirror blank. A lot of comments i've read so far indicate that the shape at the end of roughing out can be pretty much all over the place (as long as its not badly astigmatic), which makes me happier about the accuracy of the tool i made. I guess the question is whether to use steel or tiles as a tool surface? Unless anyone comes up with dire warnings of impending disaster i think i'll have a go using the washer dumps even though i still have in the back of my mind that it may not end up at the focal length i was aiming for unless i get the tool spot on. Still, i can always build the telescope around the mirror i end up with! I guess it depends on how much the steel tool will wear (not very much from what i've read) relative to the glass. Anyhow, i'll be using plate glass for my first effort, and if it goes wrong i can always make another tool and regrind using more proven methods. I'll post again when i get into it and know if its working.
Seeyahs!