The focal ratio is what you should be concerned with, i would go maybe f-8 or higher f# at this thickness, when you say grinding paper i interpret as "sand paper" and if this is the case and its all you have then burn it, the paper turns to ash and the silicon carbide which has a VERY high melting point is still there. i would say slump the blank but you may not find the facilitys and equipment to do this where your at.
we americans are far too spoiled and picky about everything,you will enjoy your telescope and making it especially! good luck.