Unless you are using something published and approved by the aircraft manufacturer, any online tool will just be some curve fits to POH data.
POH data, calculated through on a PLOG, using a CRP supplemented by a pocket calculator, should be as accurate as can normally be achieved.
Some online tool may well be more precise, but that is quite diffferent to accuracy. It also may well contain errors, not having been through the checking process that goes into a POH.
With regard to fuel flow, pretty much all aircraft fuel flow gauges are very innacuraate. I'd uae the POH numbers for primary planning and any cockpit gauge as a check for gross errors, unless I'd done some careful checking first.
G