You have worked out the QDM you are on and the one you want, so I imagine you have no problem with that.
A good rule of thumb with a QDM is to turn the opposite way to the QDM you want. eg you are on 330 and want 020, the opposite way is back past 330. So, for the 90 degree intercept you are asking about, the intercept heading required, is 020 - 90 = 290 degrees. If you wanted to intecept a QDM of 310 at 90 degrees, turn opposite way, ie 310 + 90 = 040. You can step this down as required. (Visualise it as the QDM you are on as being between the QDM required and the intercept heading required - works either way).
You don't ask about QDRs but for completeness, the technique is similar. Work out the QDR you are on, and this time turn further past the QDR required. eg on a QDR of 360, want 040, at an intercept of 30 degrees, turn past 040 by 30 degrees, ie 070. Or intercept of 90 degrees, then 130 degrees etc. Again, works either way.
To get this fixed in your mind, draw a compass rose on a piece of paper and try out the above with various scenarios.
Fortunately NDBs are being phased out. Very soon the only good thing about the ADF will be to listen to the music!!
There are as many methods as types of coffee, but I hope the above helps. Good luck!