The best altitude to fly is always determined by two things: desired level of detail and safe flying height. You are talking about collecting data on a very hilly area so that will complicate things.
You need to plan your flight for collecting 60% overlap at the highest point if you are flying at one altitude.
If you can plan your flight to stay 90m above ground that would be great. If you can do that and get consistent overlap our system will process it nicely.