There is no need to change the dose of test.
And what's the point of a 20 week cycle? Why even come off at that point? THe whole point of cycling is to give your body a good chance of recovering. You are running a cycle more than double the amount of time I'd recommend. THere is simply no need for guys who wish to maintain natural function of their HPTA to run such long 'cycles'. You may as well begin a blast and cruise phase if youre comfortable with 20 week cycles.
If you cant gain 20 pounds on 750mg test 400mg deca AND var AND winstrol over 8 weeks, something is wrong. For this reason Id also recommend nandrolone phenylprop instead of decanoate. Long esters simply arent conducive for cycling.