Yes I always use the tire mfg data rather than a generic calculator.  I
learned this years ago when I was autocrossing a '77 rabbit.  I would
scrounge used tires to use.  I believe it was a pirelli P77 in size
175/70-13 that I found that had a very small sidewall despite being called a
70 series tire.  It looked a lot more like a 50 series tire.  One thing I
look for now in buying tires for the van or my Golf TDI is the revs/mile.

Edward