i'll use easy numbers to make it easy (for me
).
best glide speed clean = 90kts. that's 1.5NM/minute. ROD is 500fpm. from 1000' agl, that's 2 minutes before you're on the ground. you have travelled 3NM.
best glide speed with flap = 60kts. 1.0NM/minute. ROD is still 500fpm. still 2 minutes til you're on the ground, but you've only gone 2NM.