Here's how to calculate what I think you're asking:
First, if you intend to invest $10,000 in HGT, you would buy units(shares). If you had bought at the closing price on Friday ($23.62 per unit), you divide $10,000 by $23.62 to determine the number of units you could buy with your money.
$10,000 / $23.62 = 423.37 units
but lets simplify it to keep the math simpler and call it 400 units with commissions and a little change back.
So now we need to find the monthly distribution, which will change a little bit from month to month, depending on the price of oil and the amount of oil actually pumped by the tenants.
The distribution for March was $0.113 per unit.
And so, to find the dollar amount of the distribution, we multiply $0.113 by the number of units.
400 * $0.113 = $45.20 in distribution payouts for your units for the month of March.
Next, to determine the annual rate of return, we multiply
$45.20 * 12 = $542.40 per year
(Now, since the distribution changes a little each month, it won't be exactly this amount, it might be a little higher or a little lower)
Finally, to determine the rate of return, we divide the payout by the initial investment:
$542.40 / $10,000 = .05424 = 5.424%
I hope this helps you with your question. If there is anything more you are still unsure about, please ask. I'm sure I or someone else will be able to help out.