At 17.5 x 20 you would have years of plenty of room between trees. Space like that could be used for intercrops of shorter term plants or trees like banana, papaya, guava, etc. Some even try planting close and removing every other mango tree but others I've seen try that became attached and couldn't bear to cut out 1/2 their trees. There is a lot of work involved in preparing land, setting up irrigation and all to put in small trees so widely spaced, it seems like using that space for something beneficial would make sense.