Oddjob62 said:
Of course... but it's a good way to get a rough idea. I've only been fixed for a couple of weeks now (and only really been cycling properly for 8 months). Have started off with 67". A lot of my journey is quite flat, but i've got one tough hill. Can get up ok with 67 so might bump up to 70 in a few weeks (I've always been a grinder anyway).
Id stay on the 67 if i were you. Spin that gear more on the flat and you will get fitter and your legs more supple. Then go for 70" if you really want.
Im sort of thinking of gearing back down. I was riding 65" and could get up to 38mph on it down a hill, sprint for signs doing 30mph and get averages of 19mph.
Im now on 72", my top speed is just under 39mph, and when i sprint for signs im not any faster.
On sunday i went down a hill i used to go down doing 38mph on a 65" gear, and i went down it doing 36mph

Ive lost some leg speed and suppleness.
Stay with 67" for abit longer before you change to a bigger gear.
I used to be able to go down hills faster or at the same speed as someone on a 74" fixed that had huge legs and was very very powerfull. Because his gear was big, he didnt have the suppleness to spin the gear.
He was no faster up hills either, even with his legs being huge and him having a bigger gear. Only time he was faster was on the flat, he could sit at a slightly higher speed for longer then i could, but i could still sort of keep up.