What malt are you using. Weyermann Beechwood malt you could go as high as 100% as others are saying. If you're using peat malt, which isn't usually used for traditional rauchbier, I would say keep it in the ounces range. Peat is quite strong, so you may only want 2-4 oz. To be honest, I haven't used much peat malt.
Is this something you're planning to enter in a competition, or just for personal consumption. The reason I ask is for competitions you'll want to error on the higher percentages. Smoke categories are sort of like judging IPAs where the judges' taste is overloaded after the first couple of beers. If your beer is judged later in the flight, it needs more smoked flavor to be perceivable. However, hopefully the judges would see a Czech lager and judge that first or second. So in reality, you probably don't need to error on the higher percentages for this particular smoked beer. It wouldn't be fair to judge a Rauch Czech lager that after a Rauch double bock. If focusing on personal consumption, then I would go with 15-20% so that it accents the lager and doesn't overpower.
I found that when I used my home smoked malt (2row with cherry/maple) it was more intense than compared to Weyermann Beechwood malt. I'm speculating this is because my LHBS buys the malt and it sits for some time. So the Weyermann's smoke is not that fresh. Take this with a grain of salt, I have no education when it comes to the properties of smoke flavor & food.
Czech lagers are light and should be consumed somewhat fresh. So you're marrying the light, crisp czech lager with robust smoke flavor. Personally, I would go with the smaller % of smoked malt. If I was making it, I'd start around 10-20% (1-2 lbs). Remember you'll have to drink 5 gallons of this stuff. Plus it gives you a reason to do a second batch based on this batch.
One thing to point out, if it does come out too smokey. Just let the beers age. The smoke flavors do mellow out over time.