Pitched roof, fibre cement slate - is this acceptable work quality?

[chappers]

9.1 " or 9.5" ain't going to matter one bit, with the bigger slates you can have a bigger gauge for the same headlap or increase the headlap for better weathering.
Just measure the gauge i.e from the bottom of one slate to the bottom of the one above, if those are 300mm soakers up the abutment then it all looks about right by eye.
According to that table the correct gauge is between 9.8 and 10 inches so if you have 5 equal gauges there at 9.1 inches then your home and dry

 

[S1llyB1lly]

Hmmm...I just gone and done the measurements as per pic and each course is slightly different! It isn't that noticeable to the eye as lots of tiles, but the tape measure doesn't lie. The soakers are 330mm x 100mm x 50mm.

 

[chappers]

perfectly normal for the top two courses to be like that if you just gauge up and see what's left rather than setting out by working out the gauge in advance and dividing it over the number of courses, not a problem with slates as you can cut to any size.
But no reason why the other courses should all be different, your headlap on the second course from the top is only 7cm(70mm) when it should be at least 100mm (60-27-26) according to the table you found. The reality is it would probably be OK but is pretty shoddy work for such a small roof.
The correct gauge on those four courses would be 245mm which would give you 110mm head lap. You would always expect a few mm variation but you have up to 50mm variation there. As I said pretty shoddy for only four courses of slates.
Get the roofer who did next doors dormer looks like he's done a tidy job there, particularly the close cut mitre on the corners