Hi Steddy
Springs do vary in stiffness even around the same weights but it is a pretty good bet to go for your weight or very slightly higher
So as you have found that 60kg is to soft and 79 are to stiff I would recon that 70-75kg would be somewhere around right for you
You are right about swapping them as they are now used so it is a case of buying a set
The good news is that you may want to keep the 79kg ones because as you get fitter you may want to go back on them
A lot of lads go 10 to 20 kg over their weight when they get to the point of being extreme so they will come in handy later
If you do find you don't need them you can always sell them on here in the adds bit or better take them to a few meets for people to try before they buy
Secondhand springs don't fetch much but you may find someone that has a set that are to soft for them and want to swap
If you are still unsure about the right weight get along to a couple of meets and someone will let you try some closer to your ideal weight
Jason