Break it down, then break it down further, into tasks you expect to take no more than 8 hours. Then multiply the total by 3. Seriously. When I was freelancing

Break it down, then break it down further, into tasks you expect to take no more than 8 hours. Then multiply the total by 3. Seriously. When I was freelancing, I always used this method, and 90% of the time would come in 10-15% under budget. For really big projects, break out a discovery phase before offering an implementation estimate.