I've learned that sum of runtime usage values of all blocks should be and is one.
But I don't think that GNU Radio uses 100 percent (= one) of CPU capability.
And I think that 'one' is a portion to the capability which is allowed and allocated to the GNU Radio.
In order to calculate runtime usage of each block, therefore, it can be done by multiplying usage of GNU Radio process.
That is, if python running the flow graph is occupying 10 percent capability of CPU and a demodulator block is indicating 20 percent in performance monitor, actual runtime usage of the block would be 0.1 * 0.2 = 2 percent of CPU.
(How to know usage of python? Maybe, `top` command in POSIX)
Is what I think is correct?