In response to the original question I posed…
I gave up with using an analogue reading from a Sika. Whilst you can get a reading from the Sika I did not trust it and do not think it is accurate enough.
Any inaccuracy on the flow rate will directly affect the system efficiency. And you would want that to be within a percent or so.
I got a Kamstrup 603 with an LCD display. This tells me on the device what the flow rate is.
I have found that the measured flow rate was being affected by bends and curves in the pipe when heating the DHW. The flow dropped to near zero when heating the DHW tank!