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"There are no dovetails or the like on my lathe-so I'm stuck with the magnetic base..." Maybe not if your just wanting the indicator for checking and adjusting part runout. Robin Renzeti on Youtube showed a custom built blind threaded top nut on his quick change tool post for his Hardinge HLV lathe. Above the nut itself he turned a short stub shaft sized to suit his magnetic bases snug. That would be .375" in diameter for my imperial sized Starrett snug if I recall it's dimensions correctly. Since it's then always there and ready to go it's fast to use. Drop the snug and cross bar in place, rough adjust the indicator for zero on the part, tighten that snug and adjust for final zero point with the cross slide. No extra tools needed or removing whatever tool is already mounted in the square turret or qctp as well. One of those is on my roundtuit list. As Gorge Thomas used to say, the time spent building any machine tool accessory is repaid every time you use it.
I should probably mention that I usually solve the problem with what I got - it wasn't a complaint ....But that you for suggesting yet another possible solution.Per