Tuning-by-guesswork: Amal main jet size?

[clanger9]

Does anyone have a suggestion for roughly how much the main jet size should increase with a free-er breathing inlet & exhaust?
My 250CSR has an open bell mouth, 389 Monobloc and megaphone exhaust (with some mild baffling). It appears to have been moderately tuned (flowed head, nothing especially radical).

The bike runs well, but the 'roll off' tuning method suggests the main jet is a bit undersized (i.e. it doesn't misfire flat-out, but it does pull slightly stronger at 7/8 throttle). Also, the nose of the plug also looks slightly hot, again suggesting it is lean on full throttle.

I've got a 200 main jet in at the moment.

Where should I start with the main jet? 210? 250? 300??
I realise it's not possible to give a definitive answer, but at the moment I don't have any feel for how quickly a Monobloc responds to a change in main jet...