Hey Baz, like someone said earier, that flow rate maybe a bit low for the entire house. Depending on the bath(s), if there's a dishwasher and washer in the house. All has to be factored in. Then there's Maxium water flow and realistic water flow. Here's a guide from Takagi. We install lots of those and have on on my own house.
https://www.takagi.com/help-me-choose/
Another from I found off the net
https://learnmetrics.com/what-size-tankless-water-heater-do-i-need/
If you need further help, send me the info and I will tell you the flow rate needed.