There is no right answer. How do you check if you are on the right track? Why is this important? The terminal value as a percentage of firm value could be anywhere from 50-80% usually. The terminal value as a percentage of firm value could be lower or higher under specific conditions too.
Look at the terminal value as a percentage of firm value to make sure your DCF is not too dependent on future projections of terminal cash flow. The terminal value as a percentage of firm value could be anywhere from 50-80%. If your terminal value is higher than 80% of the firm value, it will be considered on the higher side and you will need to explain why.
The terminal value being higher than 80% of the firm value does not make the valuation wrong. You should have a clear and realistic explanation for the situation. An example could be if the strategy of the firm is to price low to capture market share or the strategy is to become a platform before harvesting revenues, etc.
Similarly, if your terminal value is less than 50%, you must have a reason. An example could be that this business has dying revenues and therefore the terminal phase is not indefinite providing less value than usual