Ruler type level indicators for tanks

[MechP]

Hello everyone
I'm looking for some guidance for marking the tank level for a floating type level indicator. The indicator that was provided does not include the ruler itself; only a marker that slides vertically (see attacjed file). We are considering on painting or attaching markers on the side of the wall of the tank instead of providing an external ruler.
Is there a standard as per API or NFPA for this topic?
The tanks we need to mark are built as per API 650, capacities range from 1000 to 5000 bbl and services include crude oil and water.
Thank you

 

[LittleInch]

Nothing attached or inserted as an image ( which is better)

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MechP]

Thanks LittleInch. I uploaded the image

 

[JStephen]

There is no real standard.
For accurate gauging, a tape-style gauge like the Varec 2500 is commonly used, or various radar gauges, etc.
For is-it-full-or-empty visual observation, gauges similar to that shown are commonly used. There is a Varec 6700 gauge with target, for example.
I think the 6700 is calibrated in inches. If the "ruler" part is back behind the pointer, then readings will vary some depending on your distance from the tank.