LS 15 0.2mm leveling into Leica infinity

Hello firstly sorry for my Russian I will write in English
So situation is like this I have leveling line about 500 meter, have installed 2 reference benchmark to the start and end of the line it is open line and not close line it's not poligon. I know start Elevation and need to give elevation to end point, Have very precise level and invar ronds there is no wind an not too rainy or not to sunny so I can take precision when measure by using line level software. How to give precise elevation last point. instrument is very precice and I use BFFB measurement with changing level after BF and after do FB again so I make very precisely instrument is calibrated but I know it need some processing After measurement and how to get the last result can anyone tell me?
thank you in advance

 

Лет ми спик фром май харт.

 

thank you for your help but there is when you have 2 control point and I just know 1 point elevation and What can I do in this situation?

 

I quess, you should go back to the starting point.

 

so you suggest to close the loop yah? there is no other way it is necessary yah?

 

yes