As Terry said, spill holes (AKA Apex Vents) will increase stability --and up to a certain point, performance as well. How you'd size them entirely depends on the Cd of the chute. Flat chutes --like the plastic jobs that come in Estes kits-- are poor performers with Cds in the range of 0.65 -0.75. Gored --shaped-- chutes will typically have a Cd of 1.3 or better, and usually include an apex vent. Manufacturers will be able to tell you their recommended sizes, given the weight you wish to recover--and the speed at which you'd like to recover it. If you consider a flat chute "standard," you may be surprised at how much smaller of a chute you really need, on comparison...
Later!
--Coop