On the SAAMI drawing, case belts are from 0.212-0.220" from the head. Chambers are 0.220-0.227" from the breech face. Your first step is to set your cartridge upright on a flat surface, like a surface plate or a scrap of plate glass, and use the step at the end of the caliper head to measure how tall the front of the band is from the flat surface. That number plus the thickness of your shim would be your headspace, and it should fall between 0.220" and 0.227".
Personally, I would use shim washers or flat shim stock, so you don't have something as compressible as tape. With a solid steel headspace gauge, you stop as soon as you feel any trace of light contact of the gauge with the headspace determinant (the belt recess in the chamber for your cartridge). This is because the leverage from the bolt closing is high and can easily stretch the chamber several thousandths. Gunsmiths commonly disassemble the bolt to prevent the ejector from fooling their sense of feel about that initial light point of contact, thus getting an accurate result. With the tape, that is less easy but not impossible to do, but then you have to measure the tape thickness with that same delicate touch.
Because the corner of the belt on your case is less likely to be as sharp as the headspace gauge (no corner), you may also want to color the case with a magic marker to see where it contacts the chamber's belt recess to better see where to measure its height.
As others have commented, you can also elect to headspace on the shoulder for reloading. In this instance, you may find it useful to have one of the Larry Willis dies that squeezes the case inward all the way down to the front edge of the belt before using a conventional die to set the shoulder position. It may also be unnecessary. It will vary somewhat depending on your load pressure and the particular chamber you have. In the extreme, some folks have turned filler washers that extend the chamber's belt headspacing surface rearward to force the headspace to match a lot of brass they have. This achieves zero head clearance for the initial firing and before moving to headspacing on the shoulder. It saves thinning the brass in front of the belt by stretching during the initial firing.
