I didn't understand the "interpolation" part of your message.
Am I correct in the following?:
Result 1
Volume*Filling = 300
Result 2 from SOUNDING:
Exact value = 300, XCoGlo = 1.3
Lower values = 298,297,296 ;XCoGlo values 1.2 , 1.1 , 1.0
Higher values = 301,302,303 ;XCoGlo values 1.4 , 1.5 ...