Definitely follow the mftrs recommendations. My LHBS didn't have the normal yeast nutrient, so I bought this Wyeast Beer Nutrient Blend. It seems to work well on wine also but the recommendage "dosage" is only 1/2 tsp per 5 gallons!!
So yes different manufactures and types of nutrient may all call for different amounts.
But, yes, as the others have said, just multiply the ingredients by 5, but as long as that by 5 is multiplied by what the instructions on the pack say (nutrient, energizer, tanin, etc.)
I'd like to see the recipe for this too. I am willing to bet that after reviewing it, we will all recommend a larger amount of petals than is called for. Just a Jack Keller forethought. (we love ya Jack)
I'll bet Lilac wine would be wonderful.
Keep us posted.