So I had a bike fit on my Dolan Preffisio 58cm and it fits me perfectly (6’1″).
It has…

57cm TT (EFF)
150cm Headtube (40cm spacers!)
74 seat tube angle
100mm Stem

Size wise would a 56cm Kinesis Aithein be miles off with maybe a 110mm stem and a few less spacers?

57cm TT (EFF)
170cm Headtube (20cm spacers?)
73 seattube angle
???mm Stem

I’m thinking the 73 degree seat angle of the Kinesis would shorten the reach a bit?

I know people always say “get a test ride” but I’ve not test ridden any bike I’ve ever bought and gone off manufacturer height/size recommendations. Now knowing what figures suit, doing a comparison like you suggest normally works out about right.

I’m 5ft 10 and run a 56cm Aithein with a 100mm stem currently.

From what you suggest, the figures look pretty similar. Though I’ve always wondered about seat angle – surely the effective seat angle depends where you put your saddle?

Seat angle won’t matter. One degree change is about 0.7 cm on the saddle rails. Set saddle and reach to bars will be determined by TT length plus stem. The 73 degrees will, with the same saddle position relative to BB, shorten the TT by 0.7 cm. Each cm of spacers is another 0.3 cm off the stem too (1/Tan(73)).

Head angle is more important than seat angle as this also changes TT length and stem.

For example, My TCR is 55.5 TT and 73 degrees and 100mm stem. My Defy is 54.5cm TT and 72.5 degrees and 115mm stem. Position is identical.