Airline value basically takes into account all your assets - planes and cash you own. As your planes age, they will decrease in value, however this should be offset by their profits. You will note that if you lease, as more of your money goes out of your company, you will have a lower value. However, leasing can allow you to expand faster, i.e. have a greater DOC, and so may lead to a higher value in the long term.
Regarding calculating the DOC and profits of others, that can be tricky. To get a good DOC estimate, you can try comparing their airline values just before and just after the day change (i.e. on the hour). You subtract the first value from the second to get a rough DOC. Using Excel and copying the table directly out of the game can make that easier. However, you should consider two things.
Firstly, if they happen to be spending/gaining money at that time, you may get an incorrect amount.
Secondly, and more importantly, this does not take into account their monthly costs/gains, like maintenance or leases. This is not so much a problem with the method, but with the use of DOC as a measure of profitability. In the case of someone who is leasing, they have both high monthly costs (when lease revenue is due) and a high DOC. Conversely, someone who is leasing out a number of aircraft has little day to day income, but gets a large amount of money each month.
To overcome this, you could use the above procedure spaced out over a 24 hour period, i.e. gather your data at the same time on two consecutive days. This has the problem that the other airlines may do things which vary their value in the short term. For example, placing an order with a broker temporarily reduces your value, because you give money to the broker, but don't have any physical planes yet. This value will come back when the planes are received, but may cause problems for calculating monthly profit.
An even more complicated approach could work thus: calculate their DOC as above, then calculate their monthly costs by comparing their values on the last day of one month and the first day of the next (remembering to subtract their DOC earnings for that day). Then multiply the DOC by 24 and subtract the monthly costs. This approach involves more work, and requires you to be playing AM as the month changes (which simply may not be possible), so it is by no means perfect.
In the end though, it doesn't really matter what the other players DOCs are. If your aim is to overtake them on the rankings, just try to run your airline as efficiently as possible, and make as much money as you can.
