Shepherdless: You are right that there are three "opportunities" per year to get baptised.
So if the rate is e.g. 0.5% at any three "opportunities" then obviously there will be 1.5% (0.5% x 3) in the given year.
So if Exeter at 0.21% were extrapolated the X3 number for all of the circuits in its catchment area would be 0.63% in the year.
I remember an Exeter R.C. last year (2016) where the baptisms were (from memory,) something like 14 from 3,500 in attendance so 0.4%. This years figures on attendance and baptisms are well down - but bear in mind that although the percentages hold good as being a significant decrease the WT hiked off about 500 publishers from the Avon Circuit (Bristol area) and sent them to a different convention in Cardiff this year.