According to @Dr_Alex_Harrison 10-14% concentrations work best. Perhaps you can start with 100g carbs per 1L and see how that goes? Also, don’t forget about sodium.
Personally 14% has worked fine for me on the bike. During a run at the weekend I filled one 500ml bottle with a concentration of 40% carbs and used another 500ml bottle of plain water to chase down the thicker solution. That got me my 100g/hr and worked great.
The following threads contain some really great info: