Ok, I got this figured out. I wrote to that office in Geneva for preliminary calculation and the formula they used was

round(<number of full years of work in CH> / 44 * 2370) * 12 * B20(current age)

Here 2370 is the maximum possible monthly AHV pension for full 44 years of contributions, it's 2370 Fr as of 2020. And this is assuming a good paying job, >86k, low earners get less than 2370 Fr. It then gets prorated linearly according to actual full years of contribution made and rounded. Multiply by 12 (months in year) and by the factor for the current age at time of withdrawal from table B20 (or B21 for female) from https://sozialversicherungen.admin.c.../6130/download

The other limiting factor is the actual amount of contributions made, both employer and employee parts. They won't pay more than you and your employer paid them.

Wrong kind of pension