I seen a version of this a few years ago and just recently made one. I found it a great help for determining an even thickness of slip
RORY
Thats a great little invention!!! I have the excact same capilipers as you do so im going to try it 
Isnt it just! Me too! Cheers Rory.
You can buy more expensive calipers like that ready made but what you’ve done is a lot cheaper. Cheaper still is filing a slot in a washer, of course. But calipers can be had for not much these days.
Kevin, Like that idea of a slot in a washer.
I use a washer with a 12.5/13mm hole to pass the slip through to check for even sides. There is a plastic
carbon fiber composite thickness gauge that can be bought cheap (China)on evilbay. I just shaped the guide to a fine tip
and this gives a centred digital readout.
http://www.ebay.com.au/itm/Portable-Thickness-Gauge-Meter-Tester-Micrometer-0-12-7mm-Measure-TSQUA-0127-/161772654223
Great idea for direct reading of actual thickness.
If you’re thinning down to a target thickness, a pencil gauge might be another easy-to-make and easy-to-use solution. This page, http://www.derekroberts.co.uk/making/finisharch.htm, illustrates the use of a pencil gauge to mark thickness contours on a violin belly.
I’ve thought about retooling a micrometer too. But the washer works. Got that idea from the latest edition of Dave Hegarty’s book.
Here’s a page with some violin pencil gauges: The Tools. Includes a commercial model for $130.