Espresso Brewing Control

I'm throwing this up here quickly as I'm in a bit of a rush to catch a flight to London for the Lunch! trade show, so apologies if it's not well thought out... but then, when is it ever? :)
I've decided it is time to start using the TDS meter on espresso, to hopefully gain better a better understanding and more control over my extractions using more than purely my senses. My first few measurements have caused confusion. Afraid I can't apologise for the geeky number-crunching - that's what this is all about. Maybe some readers will be interested though.

Andy Schecter produced the following brewing control chart a few years back. If you're familiar with the SCAA/SCAE brewing control charts for non-espresso coffee then this chart should be clear enough.

 So I made two shots just to dip my toe in the water and see where I ended up on the chart.

Shot 1 came in at around 5.2% TDS, shot 2 around 6.5%. Both 18g dry weight.
I let shot 1 run longer, so the beverage weight was 36g (a normale, at 50% brewing ratio). Shot 2 was a shorter 25g (ristretto at 73% brewing ratio).

Quandries:
1. My TDS readings are at the low end of Schecter's scale. I'm struggling a little to see how to achieve something like 20% TDS, to be honest, and wondering if my TDS meter is more inaccurate than I realised. I'll run more shots over the coming days to see what my max/min TDS range is with the beans I have and a few constants.
2. My extraction yields are also on the low side, at around 10.5% and 9% respectively for the two shots. If I raise the TDS, of course, the yield will also increase. Or if the beverage weight were greater (i.e. lower brewing ratio) that would also deliver a higher extraction yield, in theory. But that suggests that to produce a shot that would appear on this chart it would have to be a Lungo, and that can't be right. It must be possible to produce a ristretto that has a sufficiently high TDS and extraction % to marry up with the numbers Schecter used to produce this chart.

Of course, the chart could be bollocks, but Schecter has never let me down yet so I'll discount that option.

I'll keep experimenting.  My aim isn't to produce something that would tick Schecter's boxes though - I just need to understand why my figures are so low.

**EDIT** I've been informed that my TDS meter, which uses conductivity to calculate TDS, is indeed likely to be unable to give an accurate reading in espresso. If only coffee refractometers were not so expensive.  Time to shop around.