General advice is to pick a gear that you spin at 90 rpm.
is that just for going 25 kph+ on flats or uphill? -- or anything over 20kph?
can 60-70 rpm be ok? --if you have a heavier bike? no clips? are heavier than average? crappy crank/lower bracket? sprinters legs and recovery profile?
I weigh 180 on a heavy bike... I find bobbing the legs up and down super fast to be more tiring than a higher gear... is it just lack of practise?
is the real key to target pedal pressure under 10lbs (15 or 20lbs ok?) rather than an RPM rate?
incidentally, here is a link to a seldom visited power calculator that has additional features compared to the most popular. -- estimates pedal pressure.