Tyre down or rim/wheel down (size)

I’ve just started a new build and have hit my first quandary … tyre clearance.

The frame is meant to clear a 2.4” tyre – I appreciate tyre sizing is a dark art.

I have an Ikon 2.35” tyre that on its current rim (IIRC 30mm internal) sizes up as 2.4”.

All okay so far … but … a real world 2.4” when in the frame is, IMHO, tight. With my squints on I’d say under 5mm on each side at the chainstay.

I generally avoid the claggy mud but around the wilds of Suffolk it’s pretty sandy and creates a weaponisable level grinding paste that has a tendency to self clearance …

I like the Ikon on the rear so is it worth sizing down to a 2.2” and hoping the rim gets it to closer to 2.3” or rim/wheel down size to a narrower rim?

Part of the appeal of this build was to run low pressures too 🙁

Borrow a wheel with a 25mm internal rim and stick that in there, in the first instance?

Smaller tyre will obvs be cheaper than narrower rim, but 2.35in on a 25mm internal should be better than on a 30mm anyway.