calculate Line to LIne voltage between the line with smartmaic 3 phase meter ?

Yes, it is possible to calculate the voltage between the lines.

On the Dashboard, you can write any formula in JavaScript format.

See the example of this in the article

https://support.smart-maic.com/en/knowledge-bases/2/articles/84-reactive-power-var-and-reactive-energy-varh-line-voltage

can we ask you to allow user to have this line to line voltage as a function already in the system? Or a step by step instruction on how to insert the formula to obtain a new function 

Many thanks 

Create a widget on the Dashboard, in the Arithmetic Operation section, select f(x) and write the formula (e.g. for L1-L2):

Math.sqrt(d.V1*d.V1+d.V2*d.V2+d.V1*d.V2)

See also all articles on how to work with Dashboard:

https://support.smart-maic.com/en/knowledge-bases/2-main/categories/8-dashboard-and-server/articles