At present, I have the same SAAS mechanical guage, and also a Turbosmart Eboost Street electronic boost controller and it displays vacuum and boost on its LCD display (worth $500!).

I can tell you that the SAAS guage is bang on with the Turbosmart boost controller.

For my car at idle, the boost controller and SAAS guage both read around -16 to -17.

Maybe this can help determine which one of yours is more accurate at idle.