How Do I Calculate the profit of the value of my bitcoin? Lets say I buy $50 of a bitcoin for 3916.74, And then the price goes up to $4200. How Do I calculate how much I have made in profit?
As in What type of formula do I use to see how much more my $50 is worth now that the price is higher than when I bought it?
The simplest way to calculate would be:

averagecost = ((BTC[t0] x USD[t0]) + (BTC[t1] x USD[t1]) + (so on))/total_BTC

where t0,t1 and so on are different time when you bought.

The above gives you the average cost for all your BTC

Now to know the profit, use the below formula:

profitloss = (current_BTC_price - averagecost) x total_BTC

If profitloss is negative, then you have a loss.

If profitloss is positive, then you have a profit.