In tax context, the concept of a "fair rental value" is used mainly so that people do not rent for $1000 and then pay they taxes as if they have received $800. It is not about fairness as a moral judgment but rather a synonym for "market prices".
To determine the market prices, look for how similar properties are rent (or offered for rent) where you live. The difference between $1000 (if that is the current market price) and $800 is unlikely to trigger IRS attention (and you are not cheating your tax returns claiming you received only $800 either) but it might trigger your revisiting the terms of the rental contract. This can be complicated if the other party insists on the current contract which is in their favor, so try to consider what could be in that for them. Such as a peaceful ex-relationship or the sense of their spoken commitments about the duration of the contract being considered upheld or some seemingly unrelated contractual matter being resolved with you for them. Failing that, figure out when they will have to renegotiate, mainly from the contract itself.
Generally speaking you should not strive to be told the marked price by the other party of the contract; you should form an independent opinion of it. It does not matter whether they are a real estate agent. Every competent real estate agent rents under, not above the current market prices, for their own needs.
If you do try to revisit the terms of the contract, make up your mind whether you want it to be short term or long term, and put that into the contract, including what happens if there is a need for re-negotiation or if written promises are not kept.
Note that market prices evolve over time, so don't dwell too much on a rough number you have been told some time ago, but rather look up current comparisons. Look up several of those so that you can get a feeling of the spread (random factors, from your perspective) as well. Also try to look up how long they tend to stay offered on the market; the higher priced properties remain not rented for a longer time which is kind of equivalent to renting out at 0$ to an extremely tidy person, and not having to do taxes on it.
Renting to someone you personally know and who isn't likely to set your property on fire anytime soon, is an advantage worth taking into account in these calculations, if that's the case of your ex-girlfriend. Random strangers include some reckless types.