Having an idle moment, and after recently seeing different mast measurements, checked mine to find that the top of the lower band is about 710mm above sheerline.  I could reduce it to 630mm if there was a performance benefit but it's nice to have a bit more rake before the boom hits the hoop, and not being a limbo dancer, I can still manage a tack on full rake the way it is!
Is the boom height so sensitive to an 80mm difference?

The sheerline is quite awkward to measure accurately, so i'd double check before you get the saw out! It's pretty easy to guess within 5mm though.

I think there are loads of older wooden boats out there with high foredecks who have masts made for a placcy winder and are therefore out of class. I also don't think it's boom hight that is the problem. An overlong mast will throw your spreader geometry out quite significantly (The hounds and spreaders will be higher than standard) and i think you will find a performance benefit if you do (Correctly) apply the saw and then reset according to accepted wisdom.

Here's how to measure the sheerline.

Thanks Mags, I'd already constructed the plywood tool from the picture - works perfectly, even without Rod's deluxe pencil atatchment! Measured at a point perpendicular to the mast foot the sheerline wasn't too difficult to identify and mark on the king post.

I was intersted in particular if it was common practice to set the top of the lower black band at or as close as possible, to 630mm, as mine is currently  70/80mm more, although still less than 730mm.

Why would you want to lower the rig ?
Surely the higher the better?

I am also very interested in the previous question as My Proc XII (standard 5720 luff and SS mast) has potential to raise mast foot by about 3 ins / 75mm (within sheerline limits). What might I gain or lose by doing this ?

I'm sure the intention of most boats as supplied is to have the lower band as high as is permissable. 

You run into problems when you try to retrofit masts from other boats, especially is the donor boat has a very high (Chipstow Tales) or very low (Rowsell Thin Ice) foredeck. Not an insurmountable problem, but it does - as i mentioned earlier - have implications for your tuning especially spreader angles.

Ok, so lets say I set the top of the lower band at 730mm, which sounds like the optimum place.  The hounds and spreaders are at the same height as other boats. The actual mast might be a different length below the gooseneck but all the bits above it are at the right heights - the dome of the deck is accomodated by the variation in mast length.  
The shrouds go through the deck sheaves at something very close to the sheerline height, which probably varies very little, so the main variable would be the rig 'triangle' which would be specific to each design and can't easily be changed, and so is that where the spreader angles might need some tuning?

Next step is probably to compare rig triangles. Last time I checked, our mast was 20mm further forward than a new Winder, and the shrouds were 40mm closer to the mast, so a bit of a smaller triangle all set a bit further forward. None of it sounds like alot, but all of the small bits obviously add up to significant tuning tweaks. I imagine they should all be optimised for our hull, rather than someone elses Winder!

I need to get out and sail more...

Taking offsets from the mast will only work if you have an identical hull and rig to the one you've taken the offsets from. The amount that the spreders deflect the shrouds is more important, as is the prebend that your sailmaker recommends for a given rig tension.