How do i make a moving head start always at 90 degrees

For any fixtures,

In Setup Patch screen select Edit Fixtures.

Sort by function and change the default Pan or Tilt (you didn't say which) to 90 degrees.

Also if you have Position palettes created already, just set the Offset to 90 or -90 degrees.
All you Positions will track this Offset value, and you probably won't need to adjust many of them.
If you change the Default starting value, you will need to go back and adjust all your positions guaranteed.

Hope this helps. :)