I believe that the OP understands that LBYM means "expenses < income".
However, he likes to quantify it. Well, there has not been a committee to define an "LBYM factor", but it is clear that the lower the expenses are with respect to income, the more LBYM a person is.
For example, around here, a poster may declare that he saves 30% of income, and another may say that she saves 40%. We can say that the second poster is "more LBYM" than the first.
So, I propose that the LBYM factor to be defined as the ratio of savings rate over expenses. A person who saves nothing has an LBYM factor of 0. A worker who saves 1/2 of his net income has an LBYM factor of 1. A worker who lives under a bridge and dumpster-dives for food may have an LBYM factor approaching infinity.
This simple LBYM factor can be useful in this manner. Suppose we ignore taxes, inflation, investment gains/losses, we can have an easy rough order of magnitude (ROM) estimate of how long a person's working life should be.
If the LBYM factor is 1, then it means this person's income for 1 year pays for 2 years of living, as he spends only 1/2. So, the LBYM factor of 1 means "work 1 year, retire 1 year". In this case, he spends 1/2 of income.
If the LBYM factor is 2, then it means that this person saves 2 years of expenses for each year that he works. Then, if he works for 10 years, he will have saved enough to cruise for 2 x 10 = 20 years. In this case, he can spend only 1/3 of income.
When taxes, inflation, investment returns, mortgage pay down, etc... are taken into account, of course it is not so simple. Still, a person with a higher LBYM factor can still feel virtuous over someone with a lower number. It's similar to the BMI number, which does not take into account other health factors, body shape, or fat content.