If your perf gauge has an outside scale you should be able to determine the perfs okay. The cover is not of any significant value but the setting does present a nice aesthetic presentation with the foil replica. I have the Unitrade scale which I use for on-cover stamps & it works quite well. The encapsulation of the stamp may pose some issues in the removal as I have separated several similar items & you need dexterity, keen eye, steady hand & a VERY sharp cutting device (Xacto type)
It's not going to be any rarity. Thousands of these covers were made, and all stamps for them were sourced from specialist suppliers of wholesale quantities of stamps. Miller's Mint used to and may still buy and supply full sheets, but the bigger outfit (name escapes me at this time) was the one who largely drove the market. The latter is long gone or reduced in a major way. That largely explains why the market for US sheets of the last 50 years is so poor today, and why stamps like the mint certified public accounting one (#2361) are relatively expensive, being sold on in the thousands for a CPA convention mailing well after the time of issue.
I use a Stanley Gibbons Instanta perforation gauge to check the perfs of stamps on cover and it should work in your situation also. It is clear and perfect for this type of testing. I believe that Unitrade also offers a clear perf gauge and others probably do as well.