The restore height works as follows. It will pull block hashes up till the restore height and scan blocks from the restore height on. Scanning blocks is defined as scanning a block and checking for each transaction included in the block whether `rA == aR` (alternatively denoted as `D == D'`). If this "checks out" a transaction is "credited" to your address.  

Now, if you specify a restore height after the first transaction *to* your wallet, the wallet will "miss" this transaction and thus report an erroneous balance after the wallet refresh has completed. Thus, you have to set a restore height before the height of the first transaction to your wallet. In addition, I generally advise to add a margin of safety, i.e., subtract 20k blocks from the height of the first transaction to your wallet. For example, if the first transaction to your wallet was included in block `1450000`, use `1430000`. 

To obtain the height of the first transaction to your wallet, simply enter the transaction ID / hash into a blockchain explorer like, for instance, [XMRchain](https://xmrchain.net). If you don't have the transaction ID / hash, but do remember an approximate date, you can "trial-and-error" the blockchain for an approximate height. For example, block [1350000](https://xmrchain.net/block/1350000) was mined in the beginning of July 2017, whereas block [1250000](https://xmrchain.net/block/1250000) was mined in the end of February 2017. Note that each month approximately has 22k blocks (30.5*720). Thus, as quick arithmetic trick, you can take the current height and subtract 22k blocks for each month in order to obtain a proper restore height.  

Lastly, note that the wallet creation height works similar. That is, upon creation of a wallet, the wallet will use the daemon (monerod) top block height as wallet creation height (an approximation (minus one month worth of blocks) will be used if the wallet is not connected to a daemon). It will then pull block hashes up till the wallet creation height and scan blocks from the wallet creation height on.

P.S. I've added a table of restore heights that you can use in the appendix.

**Appendix:**

    My wallet was created in | Restore Height I should use
    April     2019           | 1795000
    March     2019           | 1775000
    February  2019           | 1755000
    January   2019           | 1735000
    December  2018           | 1715000
    November  2018           | 1690000
    October   2018           | 1665000
    September 2018           | 1640000
    August    2018           | 1615000
    July      2018           | 1590000
    June      2018           | 1565000
    May       2018           | 1540000
    April     2018           | 1515000
    March     2018           | 1490000
    February  2018           | 1465000
    January   2018           | 1440000
    December  2017           | 1415000
    November  2017           | 1390000
    October   2017           | 1365000
    September 2017           | 1340000
    August    2017           | 1315000
    July      2017           | 1290000
    June      2017           | 1265000
    May       2017           | 1240000
    April     2017           | 1215000
    March     2017           | 1190000
    February  2017           | 1165000
    January   2017           | 1140000