Let's try this again.
Assume you're against a generic ranged champion.
Assume they are a wimp little shit with 325 base MS that built 
Assume their movement speed items are
.
Assume you're a Melee Champion (with 340 base MS) whose only Movement Speed item is
.
Return to formula.
(325 + 45) + (370 x 0.05) = 388.5 MS.
How much MS do you need to get away from this wimpy little shit? (Assume that fighting them is not an option because 2 v 1ing wimpy shit ranged AND their support who isn't CCing you yet is a stupid idea).
MS needed = X (Soft-cap not yet applied)
X - 0.225X = 388.5
0.775X = 388.5
X = 501.3, approximated.
Now account for the soft-cap
0.5X + 230 = 501.3
X = 54.26 MS cut out by soft-cap
Total uncapped MS necessary is 555.56 MS.
Your Movement Speed with the slow applied is approximately 306 MS.
In conclusion:
If you are a melee champion with 340 base Movement Speed and
as their only mobility item,
Assuming you are trying to get away from a ranged champion with relevant Movement-based items
,
you will NOT get away without using a movement ability of some kind.
And even then, you might not get away. You would have to account for the 82 MS advantage they have for at least 1.5 seconds, and the possibility of them using their own movement ability.
Your original comment tried to argue that
is an expensive item invalidated by
's and
's passive handicap, which I have proven incorrect. The cut of 10% to the item's slow for ranged champions does not significantly reduce the item's effectiveness on ranged champions enough.
This is assuming melee champions have even been building it. The item has to be fringe viable because it's passive is exceedingly volatile. It snowballs leads hard when used appropriately by an auto-based champion and is essentially relegated to Yordles.
Stop pretending like I was arguing Teemo heavily abuses this item. He can barely use anything effectively.