Controlled Chaos (phb 103) allows you to... "Whenever you roll on the Wild Magix Surge Table, you can roll twize and use either number."
Should this be before or after you know the full effect of the surge? ie 35-36 can make you older or younger based on a secondary dice roll.
also 77-78, if you fail a save you are turned into a sheep.
As a GM with a player who loves the idea of Wild Magic and will play it either way I'm happy for the more powerful (chose after) option but I have other players that would "game" this and I would be more happy with the less powerful option.
Also, what suggestions do people have for frequency of surges?
I was thinking of soemthing like, "The first time you use your higest level spell in combat after a long rest" or "use a Sorcery Point.
Alternatively I could just make the player roll a second d20 and if they are the same then a surge happens. Maybe modify the surge dice by the level of the spell.
I'd personally not resolve the specific results until I chose which roll to take. If I had a choice between old/young and polymorph sheep, I would pick one before rolling to save or determining "old or young." Especially in the case of spell effects, if the spell has already forced a saving throw, it's too late to go back and change to a different result- the spell has already been cast.
For frequency of surges, I assume a DM will make my wild mage roll a d20 after every 1st-level or higher spell that I cast. I also assume that I will have an automatic Wild Surge the next time I cast a spell after using Tides of Chaos, but the DM can "chill" me out by refusing to make me roll, if he or she wants to speed up the game, stop me from being so "lucky," or prevent bad luck on the Surge from messing with the narrative. My working conservative assumption is that every spell will either trigger the d20 roll or refresh my Tides of Chaos, though. I plan on reminding my DM every time I cast a spell.
1: Roll d100 twice
2: Read the two results on the chart.
3: Pick the effect you want.
4: Resolve the effect.
I'd argue that a strict reading of the rules is that you roll twice on the table and then choose the option from the table, so it's "older/younger" vs "sheep" not "younger" vs "sheep".
I don't think it's game breaking to allow the consequences to be rolled beforehand, although only for situations where you're distinguishing between different outcomes - I wouldn't roll for damage first, for example.
Thoughts_My_Aim wrote:
Thanks for the feeback. I'll go with chosing before the secondary roll. I'm sure a clever player will eventually come up with something that will allow them to tweek the second roll. 
I'd like to have mechanic for the surges so that I don't have to worry about it, subject to DM amusement of course. So I'm thinking of this:
"When ever a first level or higher spell is cast, roll a d20 and add any Sorcery Points spent on the spell. A roll of 20 or more results in a surge."
So basically 5% but when the going gets tough and the Sorc is pushing itself to make the magic stronger, the surges become more likely.