The dimensions are close enough that the problem is most likely something that you need to deal with via exposure and pigment settings rather than there being something drastically off with your Z layer height.
The ideal values would be 5, 10, 15, 20, 25mm for each step in each dimension, and you seem to be between -0.6 and +0.6mm in XY, and consistently small in Z.
This is what I would expect with the combination of resin shrinkage and light bleed.
Light bleed for this part would primarily affect XY, but not Z since the part has no overhangs. This behavior seems to be reflected in your results where we see that in Z, every dimension is smaller than nominal, never bigger.
I have not printed with Spot-HT, but G+ has some peculiar shrink characteristics and also exhibits more light bleed (if you don’t add pigments) which can make it difficult for you to get exact dimensions.
There’s been a thread on this subject that would be worth reading at:
http://www.kudo3d.com/forums/topic/outside-dimensions-perfect-inside-dimensions-are-off/#post-15648
From the different resins I’ve tried, 3DM-ABS’s dimensional accuracy is noticeably better than anything else. PR48 is hopeful, but I haven’t done extensive testing. Both MJ G+ and FTD IB have their quirks (which are quite different from each other), and I’ve not been able to do better than around +/- 0.3mm with those (using extra pigments and exposure changes).