hi nicola and welcome, if you have a look in albums then the heart i made is there to give you an idea. Mine was bevelled so used more roses than if you weres putting the flowers flat to the heart frame but id still say you would need about 80 to base it like this. Another idea maybe would be to cover the base in moss,sisal or feathers and do a rose spray on it or make a row of roses to make a smaller heart shape near the outside edge?