A definite number of thou shrink fit cannot possibly correct as you have to take into account the size of valves and their rings. Same with bearings in alu cases, you have to calculate for each size the shrink diameter - or you end up with - don´t really want to write it, makes me vomit - staking the outer races seats !!!
Vincent valves are a bit close, exit and inlet, 43 and 46 mm, so yes, shrink fits for these seats are close as well and maybe only a thou different when doing the maths. As Merlin engines are mentioned here: BMW produced aero engines from 1917 and the great fuel-injected radials for the Focke-Wulf 190 fighter. So I guess they had sufficient expertise in the company to use in their motorcycle engines too. These classic bikes may have had some flaws but not in perfect fits for bearings or valve seats . So my numbers from the BMW manual are partly my bible - plus the great book by Ludwig Apfelbeck, once designer and engineer at BMW, Horex and others. I don´t look up "Tuning for speed" much, Apfelbeck is a lot more useful in the workshop really, not available in English I guess. So when using tool steel seat rings I´d stick to percentages of shrink fits like recommended in the BMW manual or at Apfelbeck´s and that would be around 7 thou / 0,20 mm or more.
This amount may be a bit less when you get modern rings like from the motor trade in my link. Top of page lists various types of material, below the recommended shrink fits. I have not had one of these yet, could be some sintered high expansion material. Look up the sizes in the files according to dimensions. So even with these alloys you find 5 thou shrink fit at Vincent sizes. So anything less would be too risky for me, I don´t want to end up with troubles like Ducatisti are used to, even more so with alu bronce rings.
Troubles can rise with too flimsy rings and alu heads with unsufficient amount of material around the seats. So do as you like at your own risk.
And no, 200 degrees C are no factor for alu heads for dropping in seat rings, another recommended number from the manual. Aero engines with alu heads run in the range 150 - 230 degrees , 170 continuous recommended. For Lycomings and Continentals 400 degrees F are best avoided, allright in the climb. But you can expect to exchange cylinder sets for cracked heads when hard used at these temps.
Vic