Do the numbers of filters add?

I ask because there was a post about adding a 20 and a 10 together.
Are there rules of thumb, like a value of no more than 30 for an 8", etc?
(trying the question again)
Also, can someone refer me to a website where the numbering system is explained?
Thanks,
Dan

A quick Google on filter+numbers turned up this on photonotes.org

Wratten numbers.

A series of numbers used to describe colour conversion and other filters, used by some filter manufacturers.

Frederick Wratten was a British inventor who developed a fairly arbitrarily-numbered series of colour filters a century ago. His company was bought by Kodak in 1912, which continued manufacturing his filter products under the Wratten name.

As noted, individual filter numbers (particularly letter grades) are inconsistent and quite arbitrary. However they do follow a general structure of categories, as follows. Sometimes filter makers other than Kodak will supply Wratten equivalent numbers, though they may use their own numbering schemes.

2-15. Yellow filters.
These filters run from pale yellow to deep yellow.

16-32. Orange, red, magenta filters.
The most common in this range is probably red 25.

34-61. Violet, blue, green filters.
80A-80D. Cooling (blue) colour conversion filters.

85N3-85C. Warming (amber) colour conversion filters.

81-81D. Warming (light yellow) light balancing filters.

82-82C. Cooling (light blue) light balancing filters.

87-106. Miscellaneous.
The 87, 87C and 89B infrared filters (opaque to visible light) are probably the most common filters in this range along with the 96 range of neutral density filters.

cf. cooling filter, decamired, filter, gel, infrared, neutral density filter, opaque, warming filter.