You are allowed to extend a discount of 100% if you so wish but it will be taxable income to the employees. When their W2 is prepared you will need to include the FMV of the rental in their wages. You would also need to make sure that you and the employees pay in the appropriate amount of payroll taxes on this value.
To be considered diminimus under Section 132 it needs to be "small" and insignificant. There is no definition of "small" - no dollar amount attached. For some companies based on the percentage of income $500 might be considered too large but if a company has $5 million in sales then $5000 might not be considered "small".
The other issue to look at is, is it the same type of discount that might be offered to the public. For example a retail store might run a sale offering 20% off so having an employee discount of 20% would not be considered out of line. If you do not offer discounts to any other potential tenants then you could have trouble justifying even a 5% decrease in rent.
The only way to get a definitive answer is going to be to request an IRS private letter ruling which has a fee of $500 to request. Not something most will want to do.
If you want to take a small percentage - I would say less than 10% and wait IRS audits you to determine if that is too large of a percentage that would be an option. Under this option however you could be subject to penalties and interest if they determine the percentage is too large and calculate that you owe taxes.
It is not unusual for a 10% or less discount to be offered in this situation so you would probably be safe but no one can guarantee that.
Hope this gives you enough information to move forward. I know it is not a definite percentage but no one can give you an absolute percentage except the IRS. If you have a specific question still please hit the need info button and ask. If you are satisfied with the answer please hit the green accept button to close out the question and allow me to receive payment.