Do you follow the same principle of chain on big/big rings and add two links for road bikes? Always do this for mtb and it seems to work ok.

I’ve done this for about 25 years and its never not worked.

Yup, although I don’t add the extra link i usualy do on the MTB, becausue the mech has less capacity and I’ve a compact chainset. Then again I never use small-small so probably could get away with it.

[edit] that’s big-big + nothing with the mech in place, some people measure it big-big without the mech then add a link to account for the mech.

I am running a compact 34-50 with a rear 11-25 cassette which i am changing to a 12-30 (riding in the dales) which i believe is the max i can do on tiagra 4601 mech so does this alter the principle at all?

I do small ring and smallest sprocket and set the chain length so that the rear mech is just starting to take up the tension.