Is it acceptable to book a flexi-return ticket for a METV when obtaining a visa from the London Thai Embassy?

Hello.

I am planning to get a METV six month visa from the London Thai Embassy next week. Their website states they require: A photocopy of the confirmed air ticket to Thailand. Which to me sounds like a one-way ticket. I’m not sure how long I will stay in Thailand for so will do a flexi-return. Do you think I will be Ok if I book my return within the 9 month time frame? Which technically is within the validity and terms of the visa...... Or to play it even safer I can book a return within the 6 Months?? Does anybody know when I should book my return for? Thanks in advance.