Is this a reasonable angle for a rear derailleur ? Just considering a purchase and unsure.thanks

[ColinJ]

One reason that I am reluctant to make my chain longer than that is that I have managed to put a triple chainset on the bike and a long chain sags a LOT in small ring-small sprocket. (It was killing me doing the local lumpiness with a bottom gear of 39/29; now I have a much more friendly 28/30.)

BTW - the chain length was determined by the method you described above!

 

[Ajax Bay]

With a triple (I have 52-39-28) and a large largest sprocket (eg your 30t), I think there's no option but to accept that the RD will not fully wrap the chain when small-small. But the drivetrain will let you know when it's small-small (eg 28/12 for you) even in the dark, and that's an immediate prompt to shift into the middle ring. In fact the trimming likely required in small to (say) 16t sprocket will give a gentle early warning.

 

[Tigerbiten]

Another here who pushed the limits of what a derailleur would cope with on my recumbent trike.
Front 48-36-22, back 9-32.
Being a bent trike all the gears where usable due to the extra long chain, no problem with chain angles.
Running small-small the chain was not quite tight, but that was ok as it didn't fall off unless I did something silly.
But it was set up so big-big didn't break anything, that's the important one.
And 22/9 was useful as it tucked the derailleur up and back when going through long grass, so less to pick out of it afterwards ......