Tracking managed funds

I have one managed fund in my portfolio. I don’t know how to update monthly or even yearly prices, without distorting performance data.

I haven’t had much help from the help team, and the Help section d doesn’t seem to mention this issue.
I wonder if anyone has any insights on how best to manage this problem?

I assume that you have this fund set up as a custom investment?

I assume that you know how to change the current price?

Why do you feel this distorts the performance?