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.
The terminal value is lower than 50% of firm value only when no/low terminal growth or negative growth is expected. And the terminal value is higher than 80% of firm value only if the firm has very low cash flows during the forecast period and expects to make money in later years. Look at the terminal value as a percentage of firm value to understand your terminal value as a percentage of firm value. Whatever percentage the terminal value is compared to the firm value, you should have a clear and realistic explanation for the situation and must be able to defend it.