Calculating percentage gain on portfolio when buying more?

This is more of a maths problem and it is a pretty begginer one that I'm not too sure with.

But how do you calculate your profit overall if it's stocks that you keep buying?

So say purchase 1 on Year 1 was 10 stocks @ $50.

And year 2 it's $100. So that's 100% gain. But what if you say buy 15 here at $100.

And at year 3 it's $150. How do you calculate the overall profit of everything?

Is it just the total you have now - the buy amount of the previous two purchases?

The standard way is to first calculate the average buy price. There may be other methods but this is what I've used in the past to calculate it.
Year 1, 10 @ $50 = $500
Year 2, 15 $ $100 = $1500
Year 3, say 15 @ 150 = $2250
Avg buy price = (500+1500+2250)/(10+15+15)
=$4250/40
= $106.25

Then calc % gain using the current price and the avg. buy price.

Overall profit is current value (40 x $) less total of purchase costs($4250).
or
((Current value - Cost) / Cost) x 100 = percent profit