I haven't got any data to hand on the FT but the M1917 6-ton tank had 15mm plate on the hull front and sides. Ref: http://afvdb.50megs.com/usa/m1917.html
Edit: Data on the Renault FT at: http://www.tarrif.net/cgi/production/all_vehicles_txt.php?op=getvehicles&vehiclesX=14
By the way - it's Renault FT not FT-17 - the French experts convinced us of this a while ago.
Regards,
Charlie
-- Edited by CharlieC on Wednesday 4th of July 2012 02:23:16 PM
The Complete Guide to Tanks & Armoured Fighting Vehicles (George Forty and Jack Livesy) gives maximum armour thickness as 22mm, but doesn't specify minimum thickness or how the plates compare. It states maximum thickness of 17mm for the M1917.