Sizing Electric heating systems

[eaton89]

I am pretty new to the plan review area and recently had an individual request to put baseboard heat in his new 400 sqft office area. We live in an oilfield area that has lots of Natural gas or propane. I havent ran across this since requiring heat load per manual J, Ashrea or other means. So what is the correct way to size this system? Do I ask for a typical heat load and convert from BTU to watts? I googled converting and a 29000 btu converted to watts was like 8900 watts (just picked a number) which seams crazy. He originaly proposed sizing per sqft from the manufactures 7.5 watts per sqft but this doesnt account for climate, construction type or quality building orientation just to name a few. Not sure what I'm doing here. I'm pretty lost. I'm way behind with my reviews and I am feeling the presure and not able to spend the time I need in the time I have (typical).