It measures about half a degree if you measure the drop and the length and divide one by the other and use a little bit of trig.
I have the same set-up, a T35 on the Weaver rail, and it is too much slope. My T35 is barely a quarter a turn off the end of the travel, at the top of the pellet flight, and another third of a turn or so at 55 yards.
One solution is adjustable mounts but with a Weaver base they fetch something like £70 so not cheap.
AJP does a rail for the AR20 and you can specify dovetail or Weaver top, and that is adjustable, and that is one route I may take.
The other option I am looking at is to screw a length of dovetail rail to the top of the AR20 Weaver rail, and pack the front end by about half a millimetre. I have a piece of rail from Terry Robb that would do the trick. If I had a mill I could machine a slope in the underside; instead I'd have to file it by hand and then finish it on a surface plate with wet and dry paper.
The only risk areas with this are making sure that the two rails can't move in relation to each other, and that the dovetail rail is true with the barrel axis. If there is any clearance allowing the top rail to slide sideways then the left and right zero will never hold. Might end up with Araldite.