610 centres are a bit wide. 450mm would have been better, it would have made the floor less "springy". Before you fix down your plywood are you going to treat the underside of it? When I did mine I simply laid out the sheets and poured out a couple of gallons of creosote and brushed it over with an old sweeping brush, left it a couple of hours then turned them over and fixed it down. Are the timbers of the base treated? What type of roof are you going for, flat sloping or apex?