Good point by nhmaf, different manufacturers produce different width tyres to fit the same wheels.
I found Avon Am21 110/90 measured 116mm and was dissapointed to find the BT45 110/90 I replaced it with was only 111mm. And surprised to find the 90/90 BT45s width was merely 86mm, a lot skinnier than the AM21 90/90 I removed at 94mm. So there's a discrepancy of about a whole tyre size there. Being narrow, the BTs feel a bit too quick on the steering but not too bad. A consellation is the BT 45s seem stickier than either metzs or the AM21s and they've been fine so far over 1000 miles. This week is the first time I've ridden them on dry roads!!!!!!! Anyhow, I can confirm at least a 100/90 is required up front if BT45s are used.