I will take a little higher difference to make my calculation easier.
At the time of buying let the NAV be 100 for direct plan and regular be 90.
Now at the time of selling if NAV of direct is at 10 percent higher I.e. 110, NAV of regular won’t be 99(10 percent higher) It will be slightly lesser than 99. Let’s say 98.5. So even if you get more units at the time of purchase of regular plan, it will still not beat the returns of direct plan.
I hope it’s clear. Still if it isn’t take some capital, divide and see how many units you get for the NAV I have quoted earlier.
In the long run, the regular mutual funds proves out to be quite costly in comparison to the direct plans. Both of these articles gives very good information on this topic, you may have a look -
So, unless there is a very special reason, it is always advisable to go with the Direct Plans, whenever possible.