Since you are using a 200% declining balance depreciation, you have to multiply the straight line depreciation you calculated by 2 which would give you $958. However, you didn't own the asset the entire year. My guess is that you bought it in March 2018. Turbotax assumes it was bought in the middle of the month and pro-rates the depreciation by the number of days you actually owned the asset. There are 292 days between March 15th and the end of 2018, so if you multiply $958 by 292/365 you get $766.40. I hope that is clear enough.
Since you are using a 200% declining balance depreciation, you have to multiply the straight line depreciation you calculated by 2 which would give you $958. However, you didn't own the asset the entire year. My guess is that you bought it in March 2018. Turbotax assumes it was bought in the middle of the month and pro-rates the depreciation by the number of days you actually owned the asset. There are 292 days between March 15th and the end of 2018, so if you multiply $958 by 292/365 you get $766.40. I hope that is clear enough.