What's the best way?

Thanks!

Well there is some tolerance there as the gas block hole is 2 or 3 times larger than the port. But one way might be to lay a ruler on the barrel and using a sharpie to lay a line out over the port.  using your eyeball make a small witness mark on the aft end of the block, this should allow you to set the angle.  Measure the distance from the hole on the block to the aft end and mark this distance off on your barrel and that gives you your longitudinal distance. You can check by blowing compressed air or something down the barrel and feeling for air coming out of the tube or block.

did you try driving in the pins?

I just reinstalled my front sight tower/gas block...I got it straight using the MK I eyeball; but then I realized that the configuration of the pins was such that it pretty much had to go on straight, once the pins were back in.  After I was done, it certainly looked straight.  

?????

I think he means that if you get it eyeballed to where it looks good, once you start driving in the pins, it will self correct as the pins go into the grooves in the barrel.  

Oh, sorry!  It's a gas block, not a sight tower, it has allen screws to tighten.

My Armalite has a removable gas block/picatinny rail...I just eyeballed it.  you could put the upper in a vise, and then use a level, I suppose.