To determine the starting height, you can use a block explorer to get an idea of what block was mined on what date.
For example, if you know you created your portfolio in July 2017:
- Go to a blockchain explorer, such as Monero Blocks
- Check the current height (~ 1,470,000 as I write this answer)
- Calculate roughly the number of days since you created your portfolio: we are in December 17, so approximately 6 months x 30 days = 180 days.
- Multiply by 720 (see note*) to calculate the number of blocks since that date: 180 x 720 ~ 130,000
- Subtract from the current height: 1,470,000 - 130,000 ~ 1,340,000
- Check the timestamp of that block on the same website: 25th of June 2017
So you can use 1,340,000 as a starting height and not miss any transactions.
*Note: I'm using 720 for the daily block emission - it was different before April 2016 so if your wallet was created before that date the calculation won't work. But for such an old wallet you may as well start from height 0.