Magnifying glass...what power, where to buy

Im about to the stage where i need a some type of magnifier so i can tell when to harvest. Where, what kind and what magnification power is reccommended? Thanks

Something like this will work very well... http://www.monstergardens.com/growing-accessories/misc-accessories.html?page=shop.product_details&flypage=flypage-vmshopgreen.tpl&category_id=93&product_id=895

- gdp