Need help gas line supply calculation

I am installing a Paloma PH-28CDVSP Commercial Direct Vent Gas Tankless Water Heater. LP I have an 11" w.c. 250,000 BTU regulator from my tanks and less than 5 foot of line to inside my house 3/8" copper tube that goes to a 3/4" rigid pipe to enter house and back to a copper tube. With that said the max input BTU's for this unit is 199,000. I have to run 12' to get to the unit and the only other appliance on this is a cooktop, no furnace, nothing else. The propane co. told me I can use the existing 3/8" tubing to supply the unit and should not see a starvation issue, I am beginning to wonder with all of the reading I am doing. any input would be great. Oh and the unit will heat water up to 180 degrees at full throttle which I will never use. (yes it is overkill but got the unit for less than half price for a residential unit of the same temp rise/flow.) Thanks

I'm no expert, but the tables in the fuel gas code show the capacity of 3/8 copper for LP at 11" wc as only 188K BTU/hr at 10 feet and 129K BTU/hr at 20 feet with a pressure drop of .5" wc. An accurate pipe sizing would require more details about your piping system, but it doesn't look like that length of 3/8 copper would supply that water heater at full demand, which is what you would size for. You can read the fuel gas code online for free at nfpa.org, you just have to set up a free account. Appendix B has some helpful examples of gas pipe sizing.