The fact sheet is this one http://www.igmaynard.co.uk/bongo/member ... pgauge.pdf
On the 1st page it says:
The intention is to scale the gauge to read from 75o Celsius to 120o Celsius, this enables the gauge to be more active over the normal running temperature range.
Based on the above I would say that normal running is indicated with the needle pointing somewhere between straight up and about half way over to "H". ("C" = 0% and "H" = 100%), the higher indication only for when the engine is working hard.Normal running temperatures show between the vertical position ( 50 % ) and to the right ( 65 % ) which correspond to an approximate coolant temperature of 88o to 98oC which reflects the thermostat opening and various low level throttle positions. A long steep uphill climb may cause the gauge to go to the 80 % position.
Compare that with a table much further on in the sheet
From the above table 98 deg C is 75% movement, vs 65% on the first page. Typo maybe but ....This is the scale in percentage of movement from the “C” cold mark to the “H” hot mark in Celcius. A healthy
cooling system should exibit temperatures similar to those shown below measured with an infra red
thermometer.
70...........80...........85..........88.............92..........100............110........Deg C
C............20%........40%.......50%..........60%.........80%............H.........Movement from C to H
From cold, the needle rises linearly to around 92o when the return thermostat opens and the temperature falls back to about 90o. Heavy use of the accelerator on a long steep hill shows a maximum temperature of 100 to105o Celcius.
Max movement is 110 deg C vs 120 deg C on the first page.
I calculate that 80% deflection based on the first page would correspond to a temp of 104 deg C (from 10% deflection = 4 deg C)
In contrast in the abive table 80% is a comfy 100 deg C.
A few degrees at the top end is the difference between enjoying a holiday and having it ruined. Knowing when to panic is quite important
What am I missing or have I stumbled across a genuine slip up?
Has anybody validated the numbers in the guide empirically? (I accept that different gauges measure different parts so would differ a bit)
Before refurbishing my cooling system my gauge could climb up to 85% easily on harsh dual carriageway or motorway hills and I have 110 ohm resistor, so that deflection is equivalent to 100% with 100 ohm resistor. Too scared now to work this through! Quite glad I haven't found a lump of aluminium underneath the engine. Good bit is that since the work I havent yet got it up to 50% (94 deg C using 2nd table and 15% adjustment for 110 ohm resistor)
. You're supposed to be able to see it going up 
and hopefully back down 
. Trouble with the standard gauge is when it does move it is probably too late. The mod suggests 110 ohms instead of 100 ohm almost as a throwaway but I think 110 ohm gives a more balanced visualisation with "normal" being upto about the half way mark (12 o'clock)
Spherical Visions