Only it is a 108 degree included angle, not 36. This applies to only a 5 groove barrel. There are blocks available for three and seven groove barrels.

The bullet is placed in the V-groove with two lands against the legs of the V. It is then measured from the top land (of the bullet) to the bottom of the V block. This dimension is multiplied by the constant noted above and then this has a second constant, peculiar to that specific block, subtracted from the result for the final solution, the groove-to-groove diameter of the barrel.

And, yes, I do have one, and use it frequently. No, I don't recall where I bought it or I would be glad to tell you.