Gematrias off by 1

One of the favourite, and often entertaining forms of drashos is the gematria, finding a significance in the numerical value of a word or group of words. The Steipler Rav devoted the back of his sefer, ברכת פרץ, to gematrios on the parsha that can blow the mind. These aren't simply one word equaling another. Time after time he will find a phrase in the Torah having equal numerical value to the phrase that Rashi uses to explain it. One of the well known rules of gematrios is that it is allowed to be off by one. What the deeper reason is for this, I do not know. However, in the הקדמה to the לקט יושר, a fascinating proof to this concept is brought from the gemara, in the name of the תרומת הדשן. It is not only pertinent to this week's parsha, it is also connected to Tisha B'Av which we  recently commemorated, hopefully for the last time.

The reading for the morning, taken from this week's parsha, begins (4:25) "When you have children and grandchildren, and you dwell long in the land..." the pasuk goes on to explain that Bnei Yisrael will commit grave sins. And HaShem vows that Bnei Yisrael will subsequently be wiped out. The gemara (Gittin 88a and Sanhedrin 38a) learns from a pasuk in Daniel 9:14 "HaShem hastened the calamity and brought it upon us, for HaShem our God is just in all His deeds..." Is it because HaShem is just in all His deeds that he brought calamity upon us? The gemara explains that if Bnei Yisrael had dwelled in Eretz Yisrael for the numerical value of the word "venoshantem" (and you will dwell long), 852, then HaShem would have had to fulfill "avod toveidun," you shall surely perish. However, from the time that Bnei Yisrael entered Eretz Yisrael until they were exiled was only 850 years. HaShem graciously exiled us early so that we would not be doomed to being wiped out. The question is, if HaShem was being so gracious, why didn't He at least wait one more year? It must be, therefore, that 851 would have been considered equivalent to 852 and HaShem therefore had to exile us two years before. From here we see that a gematria may be off by one.