With the fixed rate and subs days engine, you can:
 set a number of days to charge for at a fixed rate
 specify that each subsequent day is to be charged at a percentage of the fixed rate
A fixed rate is a simple calculation of price x quantity. E.g. 2 x product at $10 for 5 days would be $20 since $10 x 2 = $20. Factors can then be used to increase or decrease this charge.
Charging periods
For your reference, the leading rental period is listed here as 100%, i.e. the fixed rate against the product.
Set the percentage you’d like to charge for each subsequent day after the fixed rate days in the “Subs Days” field.
For example, if the fixed rate against a product is $100, your fixed rate days is 4, and your subs days are set to 10% then 6 days rental will be $100 (4x fixed rate days) + $20 (2x subs days) = $120.
Options
Fixed rate days
Enter the number of days that the fixed rate will apply for here.
Day type

Calendar day
The number of days to charge is based on whole days; the start & end times aren’t taken into consideration. 
24 hour clock
The number of days to charge is based on 24 hour periods; the start & end times are used to calculate rental charging.
For example, a rental that starts January 2nd at 11am and finishes on January 3rd at 9am would be considered two days rental when using “calendar day” since it falls over two whole days, and would be one day of rental because the end time is earlier than the start time.
Rental days per week
Use this option to determine whether weekend days are considered chargeable. If you do not charge for Saturday or Sunday set this to 5, otherwise leave at the default value of 7.
Leeway minutes
This option is only used when the day type is set to 24 hour clock. It adds a grace period or tolerance to the end of a rental.
For example, if a rental that starts January 2nd at 11am and finishes on January 3rd at 11:30am then the total rental period is 1 day and 30 minutes. This is considered two days of rental charge. However, if you set 60 leeway minutes then the same period would be considered one day of rental charge as 30 minutes is less than the leeway minutes.