Pat,
That sounds like a reasonable way to measure backlash.
Isolating the screw eliminates the effect of lost motion. It also eliminates any spurious measurements caused by racking.
It surprises me that so many people are concerned with backlash, which is relatively easy to fix or at least, compensate for. And .0002 backlash sounds great to me.
I also ordered some supported linear shafts and bearings from this seller. I asked if he could substitute 8 short blocks for the stated 4 long blocks, and he was happy to do it. At no additional cost.
Some time ago, I tried to buy some shaft support rails from another online merchant. I received a curt reply and a phone number when I emailed a request for info. Well, I called the phone number, and the conversation went something like this:
Me: Hello, I'd like to order 4 shaft support rails, the SR-16 's.
Phone Lady: Uhmmm. What company are you with?
Me: Just me. No company.
Phone Lady: ( sounding agitated ) I'll have to find the catalog numbers...
I can't find them right now. If you give me your phone #, I'll get back to you.
Me: That'll be fine. My number is 555-5555. Thanks.
Well, I waited for her call. It never came.
So, I placed my order with our friendly Chinese merchant. Not only did I get my supports, but I got the shafts, too. Drilled and tapped. And the bearings. For roughly what I would have paid for just the shaft supports. If the witch had taken my order.
Diplomacy is the art of saying "Nice doggie" until you can find a rock. - Will Rogers