It will better if you have ASTM D 341 Chart
You can purchase it online, or ask to your LAB division, they may have the ASTM book.
- PRINT the chart on A3 or A2 paper
- Put the point of kV@40(point 1)
- Put the point of kV@100 (point 2)
- both point, you can get from oil brochure
- draw the line from point 1 to point 2
- estimate the viscosity at 90 C as your wish.
Regards,
Nurudin