Carl
Although there is no such thing as a perfect estimate, there is a better formula for calculating max hr in runners. It's in 'Heart Rate Monitor training for the Compleat Idiot'. You can get it from Amazon, but note the spelling of COMPLEAT.
Anyway, using that calculation (205 minus half your age), your max hr is 188
Now subtract your resting hr
188-48=140
now we multiply 140 by the %
so 140 x .6 =84
140 x .65=91
140 x .7 =98
140 x .75=105
140 x .8 =112
140 x .85=119
140 x .9 =126
140 x .95=133
finally, to get your zones, add your resting hr on again.
Your zones are as follows
60-65% 132-139 bpm
65-70% 139-146
70-75% 146-153
75-80% 153-160
80-85% 160-167
85-90% 167-174
90-95% 174-181
Just to throw a spanner in the works, some people have an actual max hr which is up to 20bpm different from the formula calculations, so it is advisable to do a max hr test- mind you, I haven't got round to doing one yet!