Why would you need to know the top of a cb; to compare with the forecast, interest, etc.
You should be planning how best to avoid the area, never contemplate going over it. If there are red returns, or even a large area of yellow from a radar scan at your level then the avoidance margin should be increased. Red returns indicated a storm above your level, yellow, similar and / or water-ice outflow, both conducive to ice crystal icing - associated with large storms.
Use the radar to assess the shape and extent of the storm and any others around it, which way is it moving, direction of the anvil - avoid going under that. Compare these with the expectation based on the forecast, generate and assess options for action; choose the safest.
Use your time to plan ahead instead of playing with some math formula or rule of thumb.
#1, why not ask ‘why do we need to teach angle / height / range relationships in conjunction with cbs’; what bad habits might be formed, negative training.
Just as much learning value in checking altitude / range from coast lines, etc, or angle - range cross track avoidance distance which is of much more value.