Parameter Types. Inconsistent Units converting Sqare Feet to GPM

[The-Revit-MEPlumbing-User]

I'm attempting to modify an automatic sizing roof drain I found on revitcity.
I would like to Convert the Roof area In Square Feet to Gallons Per Minute. With a variable of rainfall rate in inches per hour, which can vary from 1 to 6 depending on the location. (1 / 12 of a Cubic Foot * 7.48 Gallons in 1 cubic foot = .623 gallons per inch of rain fall per hour. Divide that by 60 to get .0104 gallons per inch of rain fall per minute.)

Below is what I thought the formulas would look like. However every time I try and formulate these parameters I get "Inconsistent Units" error, because the units are not the same I assume.
To Calculate GPM
I want a GPM parameter. "type" by "Flow" (for gpm units to carry thorough the connector.)

GPM = total area * rainfall rate * .0104
ex.. 3,340 sq/ft * 4 * .0104 = 139 GPM

Next I would like to Size the connector/pipe by gpm here is the formula I had in mind for that.
Pipe Size = if(and (gpm > 0, gpm <61), 0' 3", if(and (gpm > 60, gpm <121), 0' 4", if(and (gpm > 120, gpm <341), 0' 6", if (and (gpm > 340, gpm <711), 0' 8", if (and (gpm > 710, gpm < 1311), 0' 10")))))

Also Is there a way to pull the "Area" Parameter which my family would reside in, from the project into my family to use in this formula?

I hope this isn't too difficult to understand as it was difficult to explain. Any help or suggestions would be greatly appreciated. Also attached is the family that I'm trying to modify.

thanks,
ken.

[Dimitri Harvalias]

You'll need to. Be aure the area is (measured in square units) is divided by 1 to 'convert' it to a number.
e.g. (area/1)*4*0.14
That should get rid of your inconsistent units error message

[The-Revit-MEPlumbing-User]

I am still getting the inconsistent units. I am trying to 'convert' a number parameter, to a flow parameter in GPM. Is this even possible?