For en-route climb requirements with a time constraint then a little mental maths is needed. Firstly, to use your example, determine that you can make FL350! We then need to use slightly pessimistic assumptions to make sure we acheive the target set by ATC - assume that the a/c will climb at 500' per min ( it should if it can make the altitude, and will probably do better). If you have to climb by 2000' then begin the climb before 0319.